The present invention relates to an internal combustion engine, and more particularly, to homogeneous charge compression ignition (HCCI) engine for use as a stationary engine for a private power generator and a method for operating such an engine.
Conventional engines may be categorized into two types, a spark ignition (SI) engine and a diesel engine. The thermal efficiency of the SI engine may be increased by causing the air-fuel mixture to be lean. However, there is a limit to the concentration ratio at which a spark may propagate. Thus, an SI engine requires adjustment of the amount of air with a throttle valve. As a result, the thermal efficiency of the SI engine is inferior to that of a diesel engine. Conversely, a diesel engine has satisfactory thermal efficiency. However, the diesel engine does not sufficiently mix fuel and air. As a result, NOx tends to be generated due to local combustion of fuel at high temperatures, and soot tends to be generated due to local enrichment.
In comparison with such engines, a homogeneous charge compression ignition (HCCI) engine premixes air and fuel. Thus, the possibility of local high temperature combustion or enrichment is small, and the generated amount of NOx and soot is subtle. Further, in a HCCI engine, chemical changes cause ignition. Thus, the dependency on the concentration ratio is lower than that of an SI engine. As a result, the HCCI engine is capable of causing air-fuel mixture to be significantly lean, while achieving thermal efficiency at the same level as a diesel engine. With such advantages, HCCI engines are receiving much attention. However, in a HCCI engine, excessive heat would result in sudden combustion, and insufficient heat would result in misfires. Thus, in comparison to other engines, misfires, knocking, and pre-ignition are apt to occurring more easily. This tends to narrow the operable range of the homogeneous charge compression ignition engine.
Japanese Laid-Open Patent Publication No. 2000-64863 describes a four-cycle engine that has low NOx emissions and reduces the amount of hydrocarbon (HC) emitted from the exhaust gas. The engine includes a variable valve actuation mechanism that selectively switches the valve timing of an intake valve and an exhaust valve in accordance with whether the load of the engine is low or high. When the engine load is low, as the load decreases the valve timing is set so that the exhaust valve closes at an advanced timing before the piston reaches top dead center during the exhaust stroke. When the engine load is high, the valve timing is set so that the exhaust valve closes when the piston is near the top dead center. Further, when the engine load is high, an igniter, which is arranged in the combustion chamber, ignites and burns fuel when the piston is near the compression top dead center. When the engine load is low, instead of igniting fuel with the igniter, the engine performs homogeneous charge compression ignition (HCCI). That is, during HCCI, the variable valve actuation mechanism adjusts the timing at which the exhaust valve closes to perform internal exhaust gas recirculation (EGR).
Japanese Laid-Open Patent Publication No. 2000-192846 describes a combustion controller for an engine that optimally controls the in-cylinder pressure and in-cylinder temperature, which are combustion control parameters, under conditions enabling HCCI. To stabilize HCCI, the combustion controller includes a means for calculating a target in-cylinder pressure and a target in-cylinder temperature that enables HCCI generating the required torque. The combustion controller also includes a means for controlling combustion parameters obtained from the calculated target in-cylinder pressure and target in-cylinder temperature. The combustion parameter control means includes a means for changing at least one of the in-cylinder pressure and the in-cylinder temperature to satisfy the target in-cylinder pressure and the target in-cylinder temperature. The combustion parameter control means also includes a means for calculating a target air-fuel ratio for the target in-cylinder pressure and the target in-cylinder temperature and controlling the air-fuel ratio to satisfy the target air-fuel ratio.
When performing HCCI operation in accordance with an intake air amount and fuel injection amount corresponding to the required engine load and speed, the air-fuel ratio may deviate from the target value due to a transitional state or due to deterioration of devices. When the air-fuel remains deviated, the air-fuel ratio may enter a range in which knocking or misfires occur. The deviation of the air-fuel ratio is corrected by adjusting the fuel injection amount and the open amount of the throttle valve. However, to prevent errors, such as normal overshooting from occurring, the fuel injection amount must be frequently changed in small amounts. This lengthens the time for converging the air-fuel ratio to the target value. Further, for example, with an engine of which fuel supply amount is difficult to control with high accuracy, such as a gas engine that supplies gas fuel to an intake passage with a mixer, more time is necessary for converging the air-fuel ratio to the target value. In an engine having a stable operation range that is narrow such as an HCCI engine, when much time is necessary for air-fuel ratio convergence as described above, the air-fuel ratio is apt to entering a range in which knocking or misfires occur.
The engine of Japanese Laid-Open Patent Publication No. 2000-64863 employs internal EGR to increase the temperature of the pre-mixture and facilitate ignition. More specifically, the engine of Japanese Laid-Open Patent Publication No. 2000-64863 increases the advanced amount of the closing timing of the exhaust valve to increase the internal EGR amount as the load decreases in a low load range, and decreases the advanced amount of the closing timing of the exhaust valve to decrease the internal EGR amount as the load increases in the low load range. However, the publication does not teach how to cope with deviations of the air-fuel ratio from the target value.
Japanese Laid-Open Patent Publication No. 2000-192846 explains that the air-fuel ratio has a strong influence on HCCI and that factors affecting the air-fuel ratio limit at which knocking occurs includes the in-cylinder temperature in addition to the in-cylinder pressure. The combustion controller of this publication calculates the target air-fuel ratio corresponding to the target in-cylinder pressure and the target in-cylinder temperature to control the air-fuel ratio so that it satisfies the target air-fuel ratio. However, the publication does not teach how to cope with deviations of the air-fuel ratio from the target value.
Normally, an air-fuel ratio sensor is arranged in an exhaust passage of an engine to monitor the combustion state and perform feedback control. When the air-fuel ratio deviates from the target value, an engine provided with such air-fuel ratio sensor is capable of having the deviated air-fuel ratio return to the target value. However, the employment of the air-fuel ratio sensor does not solve the problem of the long time required for the air-fuel ratio to converge with the target value.
A knocking sensor may be employed so that the air-fuel ratio is changed when knocking is detected in order to prevent continuation of knocking. However, in addition to being relatively expensive, the reliability of the knocking sensor for detecting knocking during HCCI is low.